No abstract

Background and Aims Alterations of vitamin D metabolism are ubiquitous in patients with chronic kidney disease (CKD) and a key feature of the syndrome of CKD-mineral and bone disorder. A central paradigm in vitamin D metabolism is that the kidneys are the only relevant source of activation of vitamin D for systemic effects. However, various cells and tissues exhibit CYP27B1 activity and are capable of producing 1.25(OH)2D, which could contribute to the circulating pool [1]. This longitudinal, observational study challenges the dogma of the kidneys as the only source of 1.25(OH)2D, by investigating vitamin D metabolites in anephric patients free of active vitamin D therapy. Method Anephric patients free of active vitamin D therapy were included at time of kidney transplantation. A subset had a second study visit at 12 months post-transplant. Liquid chromatography–tandem mass spectrometry was used to measure 25(OH)D2/3, 1,25(OH)2D3 and 24,25(OH)2D3. Metabolic ratios of vitamin D metabolites over precursors were used as measures of CYP24A1 (24,25(OH) 2D3/ 25(OH)D3) and CYP27B1(1,25(OH)2D/ 25(OH)D3) activity. Biointact PTH (1-84) was measured by an in-house immunoradiometric assay [2] and FGF23 was measured by ELISA (Kainos Laboratories, Tokyo, Japan). Interleukin 6 (IL6), tumor necrosis factor-Α (TNF- Α) and interferon-Γ (IFN-Γ) were measured using an electrochemiluminescence multiplex immunoassay (Human Proinflammatory 1 V-plex, MSD, Maryland, US). Two independent laboratories performed confirmatory analyses of 1,25(OH)2D. Results All anephric patients (n = 38) had detectable (>2.5 pg/mL) levels of 1,25(OH)2D, with a median value of 13.5 pg/mL (range 3.9 to 35.7 pg/ml). There was a significant correlation between levels of 25(OH)D and 1,25(OH)2D (r = 0.583, p≪0.001), with 25(OH)D-levels explaining 34% of the variation in 1,25(OH)2D (Fig. 1). 1,25(OH)2D levels were not significantly correlated to PTH (rho = 0.279, p = 0.09) or FGF23 (rho = –0.280, p = 0.09), nor to any of the inflammatory markers; IL6 (rho = 0.068, p = 0.69), TNF-Α (rho = –0.264, p = 0.11), IFN-Γ (rho = –0.001, p = 0.99). To validate our findings, anonymized samples of 17 anephric patients and 17 CKD stage 5D with residual renal function were sent to two independent laboratories. The presence of 1,25(OH)2D was confirmed in all anephric patients. Twenty-five anephric patients underwent a successful kidney transplantation and were free of active vitamin D therapy at a study visit 12 months post-transplant. After kidney transplantation, 25(OH)D-level decreased, while levels of 1,25(OH)₂D and 24,25(OH) ₂D substantially increased. The metabolic ratios expressing CYP24A1 and CYP27B1 activity both increased (Fig. 2). At 12 months post-transplant, 1,25(OH)₂D-levels correlated with levels of biointact PTH (rho = 0.603, p = 0.002), but not with levels of FGF23 (rho = –0.262, p = 0.21). Conclusion Extrarenal CYP27B1 activity contributes to the circulating active vitamin D pool and may sustain low-normal levels of active vitamin D, even in anephric individuals. While renal CYP27B1 activity is hormonally regulated, extrarenal CYP27B1 activity seems to be mainly substrate dependent.

Background and Aims Elevated alkaline phosphatase (ALP) levels are widely recognized as a marker of poor outcomes in chronic kidney disease (CKD). This association remains poorly understood, with arguments pointing to the skeletal and non-skeletal isozyme fractions as being decisive factors. The liver and intestinal isoenzymes of ALP are able to dephosphorylate and thus inactivate a number of substrates which have been implicated in the promotion of inflammation, including the bacterial endotoxin lipopolysaccharide (LPS). Present study aimed to test the hypothesis whether association of total ALP with all-cause mortality is mainly driven by non-skeletal ALP and its link with metabolic endotoxemia and inflammation. Method Laboratory parameters of mineral and bone disease, inflammation (C-reactive protein, CRP), and metabolic endotoxemia (Lipopolysaccharide binding protein, LBP) were assessed in 431 CKD patients enrolled in the Leuven Mild-to-Moderate CKD Study between Nov 2005 and Sept 2006. Non-skeletal ALP levels were estimated as residuals of the linear regression of log transformed total and bone-specific ALP. Spearman correlation, multivariate regression, and Cox proportional hazard analyses were performed to investigate the associations between skeletal and non-skeletal ALP, LBP, CRP and mortality. Data were censored at start of renal replacement therapy. Results Median age was 63 years, 54% were men, 19% had diabetes, 31% had established cardiovascular disease. At baseline, non-skeletal ALP was associated with inflammatory parameters (CRP: rho = 0.33, P<0.001) and with metabolic endotoxemia (LBP: rho = 0.32, P<0.001). Non-skeletal ALP was a significant predictor of elevated CRP (>3 mg/dL) in multivariate models (odds ratio (OR) per standard deviation (SD) increase 1.97; 95% CI 1.21-3.21; P = 0.006). This association was attenuated by further adjustment for LBP (OR 1.40; 95% CI 0.69-2.80; P = 0.3). During a median follow-up of 8.0 (interquartile range, 2.6-14.7) years 131 patients died. High non-skeletal ALP associated with increased risk of all-cause mortality, even after adjusting for demographics, Framingham risk factors and comorbidities (Hazard ratio (HR) per SD increase 1.28; 95% CI 1.05 – 1.56; P = 0.016). This association was attenuated by further adjustment for LBP (HR 1.22; 95% CI 1.00-1.50; P = 0.051). High bone-specific ALP was not associated with increased risk of all-cause mortality. Conclusion In patients with CKD not yet on dialysis, increased non-skeletal ALP (as opposed to skeletal ALP) is associated with inflammation and increased risk of all-cause mortality. These associations are at least partly driven by metabolic endotoxemia.

No abstract